1. Field of the Invention
This invention relates to a method of processing and assembling an electrode within an electron discharge device and more particularly to the method of fabricating a photocathode element to be disposed within an electron image device.
2. Description of the Prior Art
Electrodes such as photocathode elements are employed in a wide variety of electron discharge devices including television camera pickup tubes and image intensifier tubes. A photocathode element serves to convert an incident radiation or optical image into a corresponding image of electrons. A photocathode element may be disposed within an orthicon type of television camera pickup tube for the purpose of generating an electron image corresponding to the incident radiation image. The electron image is focused upon a storage target which develops a pattern of charges corresponding to the incident electron image and the radiation image. An electron gun is provided to scan the storage target with a low velocity beam of electrons. A portion of the electrons directed upon the storage target is returned to an electron multiplier including a plurality of dynode elements which repeatedly multiplies the return beam of electrons to provide an output signal.
In the fabrication of an electron discharge device as described above, the envelope of this device is evacuated and baked at a temperature in the range of 400.degree. C to remove the occluded gases from the envelope. The various metallic electrodes are inserted within the envelope and are then heated and degassed. Next, the thermionic cathode is activated. After this normal processing of the device, the photocathode element is then deposited upon a transparent faceplate of the envelope. Illustratively, a layer of a suitable electrically conductive material such as an alloy of silver and/or antimony is first deposited upon the envelope. Next, a suitable alkali is generated to sensitize the layer of electrically conductive material. Alkali materials such as cesium, sodium and potassium are typical sensitizing materials.
The fabrication of photocathode elements is a very difficult process even when the environment of the processing is accurately controlled. The environment of formation includes many aspects including the geometry of the electrodes and the envelope of the device, the nature of the transparent substrate upon which the photocathode element is deposited, the cleanliness of the electrodes and the envelope, and the thermal cycling with which the photocathode element is deposited. When the photocathode element is of a more complex type such as the tri-alkali S-20 variety, the control of the formation process becomes even more demanding if high performance photocathode elements are to be realized. It is estimated for complex photocathode elements such as the S-20 variety, that the percentage of successfully completed photocathode elements is in the order of 30% of the attempted photocathode elements. Even after the control procedures and the limits have been established for the particular environment of a certain electron tube, it is often necessary to change significantly the procedure when it is desired to deposit a photocathode element in the new environment of another electron discharge device. Again a rigorous trial and error procedure must be incurred before control of the photocathode formation process is again obtained in the new environment.
It is therefore, an object of this invention to provide a new and improved method of fabricating an electrode or more specifically a photocathode element for an electron discharge device whereby the steps of fabrication and assembling may be more accurately controlled and may be more quickly standardized for many types of electron discharge devices.
It is a further object of this invention to provide a new and improved method of fabricating and encapsulating an electrode and more specifically a photocathode element whereby the electrode may be fabricated at a point in time and stored to be incorporated into an electron discharge device at a latter time.